Sympathetic preganglionic neurons exhibit both an intrasegmental pattern, in which cells destined to project rostrally into the sympathetic chain are located rostrally in the segment, while those that project caudally into the sympathetic chain are located caudally in the segment, and an intersegmental pattern, in which the number of rostrally or caudally projecting cells within a spinal segment is determined by the segmental position along the rostrocaudal axis. Evidence suggests that preganglionic cells comprise two distinct groups, based on their pattern of projection into the sympathetic chain. First, individual cells project either rostrally or caudally into the sympathetic chain, but rarely in both directions. Second, manipulations of the paraxial mesoderm, such as application of exogenous retinoic acid or fibroblast growth factor to the developing somites, or transposition of somites along the rostrocaudal axis, differentially alter the preganglionic cell pools. These studies demonstrate that the pattern of projections from the preganglionic cell column in the thoracic spinal cord is influenced by peripheral cues early in development. We hypothesize that preganglionic neurons have an inherent identity as either rostrally or caudally projecting cells and that this identity is specified in early development by genes that regulate pattern formation. The specific aims of this application are designed to elucidate the mechanisms that specify neuronal identity in the sympathetic preganglionic cell column. The first three aims will determine when and where signaling cues that regulate pattern in this system might act. The last two aims will test the specific hypothesis that the positional information that specifies the intersegmental gradient in the pattern of projections from the sympathetic preganglionic cell column is regulated by Hox genes and the genes that control Hox expression. The specific aims are: Specific Aim 1.To determine the temporal limits of the effect of somite manipulations on pattern in the preganglionic cell column. Specific Aim 2. To test the hypothesis that rostrally projecting preganglionic neurons are born before caudally projecting preganglionic neurons. Specific Aim 3. To test the hypothesis that preganglionic neuronal axons exit the spinal cord with a temporal delay relative to somatic motor neurons. Specific Aim 4. To test the hypothesis that the distribution of specific preganglionic cell types along the longitudinal axis of the thoracic spinal cord is determined by the Hox code. Specific Aim 5. To test the hypothesis that Hox code specification of pattern in the preganglionic cell column is regulated by members of the Polycomb/Trithorax group of regulatory proteins.